Question: Which of these numbers is prime? ${15,\ 20,\ 39,\ 83,\ 95}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 20 are 1, 2, 4, 5, 10, and 20. The factors of 39 are 1, 3, 13, and 39. The factors of 83 are 1 and 83. The factors of 95 are 1, 5, 19, and 95. Thus, 83 is a prime number.